1. Field of the Invention
The present invention relates to a cryogenic refrigerator, and more particularly, to a cryogenic refrigerator suitable for cooling a superconducting coil.
2. Description of the Related Art
A Gifford-McMahon (GM) refrigerator or a pulse tube refrigerator is known as a refrigerator that generates cryogenic temperature. Such a refrigerator includes a valve that switches a flow of a high-pressure working gas and a low-pressure working gas, and a motor that drives the valve. Such a refrigerator is used for cooling, for example, a superconducting coil that generates a strong magnetic field.